The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
I.E.E.E. sections 802.11, 802.11a, 802.11b, 802.11g, 802.11h, 802.11n, 802.15, 802.16, and 802.20 (collectively sections 802.11), which are incorporated herein by reference in their entirety, define ways for configuring wireless networks and devices. According to these standards, wireless network devices may operate in either an ad-hoc mode or an infrastructure mode.
Referring now to FIG. 1, in the ad-hoc mode, each client station 10-1, 10-2, . . . , and 10-N (collectively client stations 10) communicates directly with other client stations 10 without requiring an access point (AP). Referring now to FIG. 2, in the infrastructure mode, each client station 20-1, 20-2, . . . , and 20-M (collectively client stations 20) communicates with other client stations 20 through an AP 24. The AP 24 may provide a connection to a network 26, a server 28, and for the Internet 30.
Referring now to FIGS. 3A-3B, a wireless network device (hereinafter device) 50, such as the AP 24 and the client stations 10, 20, generally comprises at least one antenna 52, a physical layer (PHY) module 54, and a medium access controller (MAC) module 56. Additionally, the device 50 may include a processor 58 and other components. The PHY module 54 communicates with the medium (i.e., air) via the antenna 52. The MAC module 56 communicates with the PHY module 54 and the processor 58.
The PHY module 54 comprises a radio frequency (RF) transceiver module 55. The RF transceiver module 55 comprises a transmitter module 70 that transmits data and a receiver module 72 that receives data in the form of modulated RF carriers via the antenna 52. The transmitter module 70 may comprise an encoder module, a modulator module, an analog-to-digital converter (ADC) module, (all not shown) etc. The receiver module 72 may comprise an automatic gain control (AGC) module, a filter module, an ADC converter module, a demodulator module, a decoder module, (all not shown) etc.
Sections 802.11 provide that data may be transmitted via multiple channels in a predetermined frequency band. For example, section 802.11a provides eight channels in a 5 GHz frequency band, section 802.11b provides 14 channels in a 2.4 GHz frequency band, etc. Except for section 802.11n, the channels are 20 MHz wide (hereinafter 20 MHz channels). Adjacent channels in the frequency bands are 5 MHz apart. Data rates and throughput may be increased by simultaneously transmitting data via multiple channels. Overlap among the channels may be avoided if data is transmitted on channels that are at least 30 MHz apart. For example, devices that are compliant with section 802.11b may transmit data via channels 1, 6, and 11.
Although data may be simultaneously transmitted on multiple channels, the MAC module 56 is designed to share a single channel when two devices communicate. This may decrease the throughput as the number of devices that transmit data increases. According to section 802.11n, the throughput may be improved by using multiple antennas in the device 50. Specifically, multiple antennas may be arranged in a T×R multiple-input multiple-output (MIMO) configuration in the device 50, where T and R are integers greater than zero and denote number of transmit and receive antennas, respectively. Each antenna communicates with a RF transceiver that is dedicated to that antenna.
Section 802.11n provides that data may be transmitted via channels that are 40 MHz wide (hereinafter 40 MHz channels). Each 40 MHz channel comprises two 20 MHz channels. A first 20 MHz channel is called a primary channel or a control channel. A second 20 MHz channel is called a secondary channel or an extension channel.
Devices that comply with section 802.11n are compatible with devices that comply with sections 802.11a, 802.11b, 802.11g, etc. Specifically, devices that comply with section 802.11n communicate via the primary or control channel with devices that comply with sections 802.11a, 802.11b, 802.11g, etc.
A wireless network (hereinafter network) may include devices that comply with different 802.11 sections. For example, the network may comprise the AP 24 that complies with section 802.11n. Additionally, the network may comprise some client stations 20 that comply with section 802.11(n) and some client stations that comply with 802.11 sections other than section 802.11n.
The AP 24 may transmit data via a 40 MHz channel using the entire 40 MHz bandwidth of the 40 MHz channel when communicating with client stations 20 that are also 802.11n-compliant. Alternatively, the AP 24 may transmit data via the primary or control channel when communicating with client stations 20 that comply with 802.11 sections other than section 802.11n.
The PHY module 54 may transmit and receive data in packets. When transmitting data, the PHY module 54 may transmit a preamble having a predetermined periodicity in front of every packet. The preamble comprises repetitions of a sequence of known symbols. The predetermined periodicity of preamble sequences is the same whether data is transmitted via 40 MHz channels or 20 MHz channels. That is, preamble signals are substantially identical whether data is transmitted via 40 MHz channels or 20 MHz channels. The sequence of symbols may be used for synchronization by devices that receive the data. When the PHY module 54 receives data, the receiver module 72 performs a synchronization operation called symbol timing using the preamble signals before processing the data in the packet.
When multiple devices in the network simultaneously transmit data, collisions may occur. Data may get corrupted due to collisions. Corrupted data may be discarded when received. Data that is discarded may have to be retransmitted. Thus, collisions may decrease the quality of service (QoS) of the network.
To minimize collisions, devices in wireless networks utilize a carrier sense multiple access (CSMA) protocol to determine when a channel is free to transmit data. Ideally, only one device may transmit data via the channel at a given time. The devices “listen” for activity (i.e., presence or absence of RF carrier) in the channel before transmitting data via that channel. Listening for activity is generally referred to as carrier sensing (CS). Since multiple devices perform CS by simultaneously accessing the medium (i.e., air), the protocol is called carrier sense multiple access protocol.
Referring now to FIG. 4, the receiver module 72 comprises a channel sensing module 74 that senses whether a channel is busy or free when the device 50 listens. The channel sensing module 74 generates an estimate that indicates whether the channel is busy or free. The estimate is called clear-channel assessment (CCA). The channel sensing module 74 may generate CCA based on CS and/or energy detection (ED).
Accordingly, the channel sensing module 74 may comprise a carrier sensing module 76 and/or an energy sensing module 78. The carrier sensing module 76 senses whether a carrier is present or absent in the channel. The channel sensing module 74 generates CCA, which indicates whether the channel is busy or free depending on whether a carrier is present or absent in the channel, respectively.
Additionally or alternatively, the energy sensing module 78 may detect presence or absence of RF energy in the channel. In that case, the channel sensing module 74 generates CCA, which indicates whether the channel is busy or free depending on whether the energy sensing module 78 detects presence or absence of RF energy in the channel, respectively.
In either case, the PHY module 54 communicates the CCA to the MAC module 56. When the CCA indicates that the channel is busy, the MAC module 56 concludes that another device is transmitting data on the channel and decides that the device 50 may not transmit data on that channel. On the other hand, when the CCA indicates that the channel is free, the MAC module 56 concludes that no other device is transmitting data on the channel and decides that device 50 may transmit data on that channel.
Rate of collisions and quality of service in wireless networks depend on the accuracy with which the channel sensing module 74 generates CCA. The accuracy with which the channel sensing module 74 generates CCA, in turn, depends on the accuracy with which the carrier sensing module 76 can perform CS and/or the energy sensing module 78 can perform ED.